Leaf cuticle-assisted phototransformation of isoproturon

Abstract

Isoproturon is a pre- and post-emergence herbicide used generally to manage Phalaris minor and Avena ludoviciana in wheat. The importance of postemergence and foliarly applied pesticides has been increasing. The first reaction environment of these herbicides is the leaf surface made up of polymerised esters of higher fatty acids. In the presence of sunlight, the fatty substances on leaf surfaces generate reactive radicals, or ions which may interact with herbicide molecules leading towards degradation resulting in the loss of bioefficacy of herbicides and the formation of metabolites of unknown toxicity. The present experiment was conducted with the objective to know the photochemical behaviour of isoproturon on different leaf-extracted epicuticular cutin surfaces. Epicuticular waxes, i.e. cutins were extracted from P. minor and A. ludoviciana leaf surfaces using dichloromethane as a solvent and further purified with activated charcoal. On the cutin surface under UV-light (365 nm), the transformation of the herbicide is very slow as it is evident from its higher half-life values, 75 min on P. minor cutin surface and 115 min on A. ludoviciana cutin, as compared to that on the glass surface (half-life 53 min). A similar pattern was also observed under the sunlight condition. It may be due to the quenching effect imparted by the cutin material or simply screening effect of it on the herbicide. The sunlight-irradiated extracts of isoproturon and its degradation products were analysed by LC–MS/MS using electrospray interfacing technique and the structures of six different photoproducts were characterised by their respective spectra. The degradation pathways involved mainly demethylation and ring oxidation processes forming hydroxylated compounds. The mixture of these photoproducts were found to be non-toxic to A. ludoviciana and P. minor.